1. Field of the Invention
The present invention relates to a piezoelectric ultrasonic motor, and more particularly, to a metal tube type piezoelectric ultrasonic motor having a piezoelectric element arranged on the exterior of a metal tube.
2. Description of the Related Art
In general, a piezoelectric ultrasonic motor includes a piezoelectric stator having a piezoelectric element attached thereto and a rotary shaft contacting the piezoelectric stator to convert oscillation energy of the piezoelectric stator into rotation energy through friction against the piezoelectric stator. The piezoelectric motor is classified into several types.
As an example, U.S. Patent Application Publication No. 2005/0052098 discloses a metal tube type piezoelectric ultrasonic motor.
As shown in FIGS. 1A and 1B, the metal type piezoelectric ultrasonic motor disclosed in this document includes a hollow metal tube or housing 14, four (4) piezoelectric elements 18, 20, 22 and 24 mounted on the exterior of the housing 14, a shaft 12 received inside the housing 14 and having threads 17 formed on the outer periphery thereof, a nut 16 with the inner periphery meshed with the threads 17 of the shaft 12 and the outer periphery inserted into an inner space of the housing 14, and a bushing 28 for supporting the rotation of the shaft 17.
In this piezoelectric ultrasonic motor 10, the piezoelectric elements 18, 20, 22 and 24 generate strain when an electric field is applied thereto, which is transferred through threads in the inner periphery of the nut 16 to the threads 17 of the shaft 12, thereby to rotate the shaft 17.
However, since the conventional metal tube type piezoelectric ultrasonic motor 10 produces rotational force through linear contact between the threads of the nut 16 and those of the shaft 12, rotation number and torque are low and thus sufficient power cannot be achieved.
Furthermore, the shaft 12 of the piezoelectric ultrasonic motor 10 is moved into or out of the metal tube in response to the rotation of the shaft 12, which needs to be supported between the nut 16 and the bushing 28. Then, the length of the shaft 12 should have an enough margin to be longer than the distance between the nut 16 and the bushing 28 (i.e., the length of the metal tube). This disadvantageously increases a mounting space for the piezoelectric ultrasonic motor.
Recently, owing to rapid development of IT industries, microscopic camera modules to be mounted on a mobile phone and the like are getting more important. To meet demands of users, microscopic camera modules having automatic focusing and/or optical zooming function are being distributed.
An electromotive motor or piezoelectric stator (or piezoelectric ultrasonic motor) can be used to drive or transport a lens in such a microscopic camera module. The piezoelectric ultrasonic motor using a piezoelectric element has been proposed as a lens driving actuator for the microscopic camera module due to its merits such as rapid response, prevented reverse driving and high resolution in transport.
Accordingly, a piezoelectric ultrasonic motor adequate for lens driving for such a microscopic camera module is demanded.